这是怎么回事呢? 不是矛盾吗
“介词＋关系代词”
介词的选择与主句中先行词的搭配密切相关。例如：
That”s the reason for which he was late for school .
This is the book on which I spent 8 yuan .
定语从句中谓语动词是含有介词的动词短语时，介词应放在动词之后，不能拆开放在关系代前 例如：
This is the key which you are looking for .
This is the baby whom you will look after.

没有矛盾.
1:当介词与定语从句中的谓语动词/动词形成固定搭配时.这时没有"介词+关系代词的结构.即介词不用提前.
2:
分析一下你所举例的句子:
1:This is the key which you are looking for .
形成固定搭配:look for 寻找
2:This is the baby whom you will look after.
形成固定搭配:look after 照顾
3:That”s the reason for which he was late for school .
这里很明显没有提前late for 的for了.而是固定搭配:the reason for ....的原因
This is the book on which I spent 8 yuan .
这里从句的谓语从词搭配:spend sth on sth.spend与on的联系明显没有late for的联系密切.所以可提前on
有错请指教
谢谢